consistently return "memory_usage" as double.

src/MISC/dump_xtc.cpp

@@ -310,9 +310,9 @@ int DumpXTC::modify_param(int narg, char **arg)
    return # of bytes of allocated memory in buf and global coords array
 ------------------------------------------------------------------------- */
 
-bigint DumpXTC::memory_usage()
+double DumpXTC::memory_usage()
 {
-  bigint bytes = Dump::memory_usage();
+  double bytes = Dump::memory_usage();
   bytes += memory->usage(coords,natoms*3);
   return bytes;
 }

src/MISC/dump_xtc.h

@@ -45,7 +45,7 @@ class DumpXTC : public Dump {
   void write_header(bigint);
   void pack(tagint *);
   void write_data(int, double *);
-  bigint memory_usage();
+  double memory_usage();
 
   void write_frame();
 };